This invention relates to an improved process of forming conductive layers in semiconductor device fabrication.
Formation of the conductive layers such as electrodes and interconnection layers in integrated circuit semiconductor devices is one of the main and important processes in semiconductor device fabrication. Such conductive layers have usually been formed of aluminum, of the purity of the order of 99%, as aluminum excels most of other conductor metals in ease of deposition and etching and thus is conveniently usable on a semiconductor device as a material for conductive layers. The use of high purity aluminum, however, has involved some serious difficulties with semiconductor devices having a silicon substrate. As is well known, the substrate is heat-treated after formation thereon of a conductive layer of aluminum in order to form ohmic contacts between the contact regions of the silicon substrate and aluminum and, in this process, there arises in the contact regions a phenomenon that part of the substrate silicon diffuses into the aluminum layer. As a consequence, small pits are left in the surface of the contact regions and aluminum enters into these pits from the conductive layer as a substitute for the silicon dissolving or diffusing into the conductive layer. In cases where the device is of the planar structure having a p-n junction formed at a relatively shallow level, the pits may reach the junction level, involving the danger that the withstand voltage of the p-n junction will be deteriorated and the reverse leak current thereof increased.
Further, in the prior art, connection of the conductive layer to the contact region of the substrate has another problem, where the contact region is surrounded by the opposite conductivity type. In detail, a semiconductor region of one conductivity type is formed in the semiconductor substrate of the opposite conductivity type or in another region of the opposite conductivity type formed in or on the semiconductor substrate of one conductivity type, and then is covered with an oxide film. Thereafter, a contact window is formed in the oxide film and a conductive layer is deposited to make direct contact with the one conductivity type region through the contact window. In this process, two separate photoresist steps are required; one for defining the position of the one conductivity type region and the other for defining the position of the contact window. Since these two steps are performed separately, there is involved the danger that the contact window be placed out of registry with the one conductivity type region, partly extending beyond the latter, with the result that the conductive layer to be contacted with the contact region through the contact window causes short-circuiting between the one conductivity type region and the adjacent opposite conductivity type region. In order to avoid such danger, it has been usual to provide the one conductivity type region with an appropriate allowance for alignment with the contact window, which causes an undesirable increase in the size of the one conductivity type region and hence in the size of the semiconductor device.